The present invention relates generally to methods and apparatuses for digitizing multiple data streams, and more particularly to a method and apparatus for digitizing multiple data streams that are geographically diverse.
Advances in analog to digital converters (A/Ds) have made transmission of the digitized analog RF return path in a Hybrid Fiber-Coax (HFC) cable system an attractive alternative to analog transmission because digital transmission relaxes the requirement for expensive linear transmit lasers. Moreover, modern digital signal processing (DSP) techniques that are embodied in reconfigurable digital hardware devices, which are known as field programmable gate arrays (FPGAs), can perform processing tasks that were previously relegated to RF devices. Examples of such functions include, inter alia, signal adding, filtering, channelizing and demodulating. While analog signal processing functions have a digital counterpart, the digitization process introduces additional flexibility and tradeoffs that do not have an analog counterpart. Simple examples would be digital word length (e.g., a number of bits/sample) and sample rate.
Once major issue exists that is peculiar to the digital return path of an HFC system. In such a system, the RF return path could be digitized in the node and then transmitted digitally to the Hub and/or Head-end. A problem occurs when two or more digitized signal streams from different nodes are to be added together, particularly if the nodes are geographically diverse, and hence are subject to different maintenance schedules and environmental conditions. Ideally, two signal streams would be sampled at identical sample rates and thus be synchronized prior to being summed.
While each node would have identical sampling clock frequencies generated from a crystal oscillator, oscillators suitable from a performance and economic standpoint for HFC may drift up to five parts per million over time and temperature. For a 100 MHz oscillator that would be used in a 5–40 MHz return path, this would be equivalent to an oscillator whose actual frequency range could range from 99.995 MHz to 100.005 MHz. The worst-case difference between two digital data streams that are to be added would be as much as 10 kHz.
Once must then consider how long it would take the synchronizing first-in-first-out (FIFOs) to underflow or overflow because of the sample rate difference between the two data streams. Using the numbers above, this can be shown to be in the 1–2 msec range, depending upon the size of the FIFO buffers. This would result in the loss of return path data approximately every millisecond, which results in unacceptable performance.
To keep the FIFOs balanced (i.e., the input data rate equals to the output data rate out), one could periodically drop a sample from the input of the FIFO to keep it from overflowing or periodically repeat a sample at the output of the FIFO to keep it from underflowing. However, unless the original RF signals are highly over sampled (by orders of magnitude), periodically dropping or adding samples will introduce an unacceptably high distortion level such that the data will be excessively degraded. Current 10-bit A/Ds can be clocked up to 105 MHz, which is sufficient to satisfy the Nyquist sampling theory, but far less than the orders of magnitude needed when sampling a 5 MHz to 40 MHz return band.
The present invention is therefore directed to the problem of developing a method and apparatus for digitizing multiple data streams whose clocks may vary due to oscillator drift.